Need Help With Langford's Flag Cancel Encyclopedia

I am learning to use Frederick Langford's Flag Cancel Encyclopedia.I am confused about the meaning of notations in column 4. For example, 17-02,17-128, 17-219, etc. Can anyone explain or tell where to find them..

Fred was intense (and quirky) in his collecting.
As it turns out, the flag portion moved around the country from city to city as machines came back for maintenence, replacement, and parts reused, etc.
The "17-xxx" notation records the movement from city to city of the flag portion.
See explantion in the 3rd edition, pages 96-97, or more extensively in the 4th edition pages 98-103.
In short, I believe most flag/machine collectors ignore this data, including myself, and I collect several flags quite seriously.

Add: As for quirky, beyond his unusual alphabetizing of the states, his list of what creates a variety or not (4th ed, page 5) clearly contains some "not a variety" listings which a specialist would readily accept as a collectible type/variety. The same is true for his Doremus machine cancel volume, where for example, a change (or alternation) in the killer from 16 to 18 vertical bars is not a variety, yet a change of the flag portion in an Amrican machine would be.

Bottom line, Langford's works are incredible research over a lifetime of study, but one can still go farther.